1. The technical field to which the invention pertains includes the art of measuring and testing as directed to fluid pressure transducers.
2. High accuracy, low range pressure transducers of the capacitance type are disclosed, for example, in U.S. Pat. Nos. 3,195,028 and 3,557,621. Pressure transducers of this basic type are commercially available from a plurality of manufacturers and typically are comprised of a circular metal membrane supported under radial tension opposite a capacitor plate and separating two gas type plenums. A difference in pressure incurred by the plenums deflects the membrane which in turn produces an electrical output from variation in electrical capacitance incident to the changed position of the membrane relative to the capacitor. In a typical construction, the capacitor plate is formed by depositing a conductive film on an insulating substrate such as ceramic or glass. Positioning of the ceramic substrate relative to the membrane support structure serves to establish the range of capacitance values generated by membrane motion.
To facilitate iteration of the nominal capacitance values and in-process manufacturing procedures, it has been found convenient to mount the ceramic substrate on the membrane support structure in a manner that is readily removable. Typically, in that arrangement the ceramic plate rests against a precision registration surface and is held in position by a combination of forces including friction forces at the ceramic-to-metal interface and a spring force extending perpendicular to the registration surface.
While the foregoing arrangement has generally functioned well to provide the expected accuracy of such instruments under stable operating conditions, it has been found to produce inaccurate indications of measured pressures when the assembly incurs changes in temperature. That is, as a result of temperature change it has been determined that a differential force is generated in a radial direction parallel to the membrane surface. Moreover, the magnitude of the problem is compounded somewhat where the ceramic substrate and the membrane support structure have different thermal coefficients of expansion. However, even where the thermal coefficients of expansion are identical, these forces are generated during thermal transients where temperature gradients exist throughout the structure.
A consequence of the foregoing forces is to induce slippage at the registration surface, as well as force couples which distort the structure as a whole. Distortion, in turn, causes motion of the capacitor plate relative to the membrane in the absence of a pressure input and is, therefore, a source of undesired output or error. At the same time, after such slippage has occurred, residual radial stresses are produced, and that in turn tends to produce a non-repeatable undesired output.
Further complicating the adverse force problem is that in transducer units having full scale below one atmosphere, a common occurrence is for the membrane to be deflected by an overload input pressure into contact engagement against the capacitor plate. The latter tends to alter the friction force relationship at the registration surface and from which slippage at the interface can eventually result.
While the instrument error produced by the foregoing forces has generally been recognized, a ready solution therefor has not heretofore been known.